class_docstr: SQN or SystemQualityNumber. Defined by Van K. Tharp to categorize trading
  systems.
  
    - 1.6 - 1.9 Below average
    - 2.0 - 2.4 Average
    - 2.5 - 2.9 Good
    - 3.0 - 5.0 Excellent
    - 5.1 - 6.9 Superb
    - 7.0 -     Holy Grail?
  
  The formula:
  
    - SquareRoot(NumberTrades) * Average(TradesProfit) / StdDev(TradesProfit)
  
  The sqn value should be deemed reliable when the number of trades >= 30
  
  Methods:
  
    - get_analysis
  
      Returns a dictionary with keys "sqn" and "trades" (number of
      considered trades)
code: def notify_trade(self, trade):
      if trade.status == trade.Closed:
          self.pnl.append(trade.pnlcomm)
          self.count += 1
  
file_path: backtrader/backtrader/analyzers/sqn.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import math
  from backtrader import Analyzer
  from backtrader.mathsupport import average, standarddev
  from backtrader.utils import AutoOrderedDict
label_desc: Use image node_id 3 for calling the SQN obj's underlying member method code with example usage: obj.notify_trade(trade) without return types